The invention generally relates to methods and apparatus for ink jet printing and plotting, but more specifically the invention relates to ink jet recording methods and apparatus, wherein
at least one ink jet is produced which disintegrates into a series of minute drops, PA1 the drops are selectively charged to determine whether an individual drop, in a recording mode, is intended to travel along a recording path onto a predetermined location on a record or target surface or is prevented to produce a record on said surface, PA1 each charged drop is deflected by an electric field by an amount depending on the charge of the drop, and PA1 relative transverse motion is effected between the path of the record producing drops and the record surface. PA1 at least one ink jet is produced which disintegrate into a series of minute drops, PA1 the drops are selectively charged to determined whether an individual drop, in a recording mode, is intended to travel along a recording path to impinge onto a predetermined location on a record surface or is prevented to produce a record on said surface, PA1 each charged drop is deflected by an electric deflecting field by an amount depending on the charge of the drop, and PA1 relative transverse motion is effected between the path of the record producing drops and the record surface, PA1 Determining any deviation between said predetermined location and an actual location of impingement of the record producing drops of the jet, and PA1 applying an adjustable bias charge to each drop at least during the recording mode, said bias charge being chosen so that the deflection of the drop caused by the action of said electric deflecting field on said bias charge carried by said drop minimizes said deviation. PA1 a. nozzle means to generate a liquid jet which disintegrates into a train of drops at a point of drop formation, PA1 b. control electrode means to charge the drops of the jet by an electrical control signal, PA1 c. means to generate an electric deflection field approximately perpendicular to the jet direction, PA1 d. means to select the drops on the basic of their charge to determine, whether a specific drop proceeds to and impinges on a predetermined location on a record receiving surface or is intercepted and prevented to proceed to said surface PA1 e. means to apply an adjustable DC voltage between the jet liquid and the control electrode means which allows the adjustment of the jet direction in a plane parallel to the electric deflection field, and bias voltage being appreciable lower than the control signal voltage which affects said selection.
Electrical controlled, continuously generated ink jets are used in many fields of industry and technics to print alphanumeric characters or images in color. In several of such applications a plurality of such jets is used simultaneously. Ink jet plotters which are used as output devices to print out color images prepared or processed by computers are a typical example of such applications. A typical ink jet color plotter comprises three nozzles which are mounted on a carriage and produce continuously three ink jets having the colors magenta, yellow and cyan, respectively, and directed towards an ink receiving surface, as a recording paper mounted on a drum, where the jets impinge on the paper in three separate, well defined locations. If the drum is rotated at high speed and the carriage is slowly moved along the drum axis by a stepper motor and a lead screw, each point of the recording paper surface is addressed once by each of the jets. By on-off control of jets by electrical signals derived from a signal source, e. g. a magnetic tape read synchronously with the plotting operation, images prepared by a computer and recorded on the tape can be plotted in color. A preferred technique of ink jet control is described in U.S. Pat. No. 4,620,196 incorporated herein by reference thereto.
In a plotter of the above described type, actually three color separations of the image in the colors magenta, yellow and cyan are printed on top of each other, thus rendering a full color image. Usually one more, fourth jet with black ink is used to enhance the color density and resolution. To achieve maximum image quality it is of course very important to ensure good registry between the three or four color separations making up the final image.
When processing an image by a computer, usually the color density of each pixel of each of the color separations is calculated in the form of a digital number. These numbers are then converted into suitable electrical control signals by an electronic control circuitry of the plotter. These control signals are then used to control the respective jets at the precise moments when the jets address, i. e. are directed to the pixel positon in question. Since the jets do not meet on the paper but are separated from each other by a well defined distance to avoid mixing of the liquid inks, a suitable delay has to be introduced between the control signals which control the ink jets for recording the individual color separations.
If the directions of the jets are not carefully controlled, the jets will not print the pixel information supplied by the computer on the same pixel position. This results in an incorrect registry of the color separations and are correspondingly debased image quality.
In the ink jet plotters presently available the registry of the ink jets is obtained by manually adjusting the direction of the nozzles mounted on the carriage. Since the nozzle and, thus, jet direction may vary slightly between subsequent plotting operations due to various causes, the adjustment may have to be carried out quite frequently. This is time consuming and cannot be effected by untrained personel. The problem of the nozzle adjustment is particularly aggravating in plotters employing more than three or four jets to increase the plotting speed, e. g. in ink jet printing machines or plotters which are intended to be used as high speed printers or to replace conventional printing machines. Such a high speed plotter may comprise 100 to 500 jets and it is obvious that in such a case a manual adjustment of each of these many nozzles is not feasible any more. Thus, it is desirable to provide a method and a device which allow the jet adjustment solely by electrical signals and further to perform this adjustment automatically by means of a suitable control circuitry.
To ensure perfect registration of a plurality (two or more) of ink jets, e. g. of the ink jets which record three or four color separations which together constitute a color image, it is necessary that each of the jets (with possible exception of one jet, which may serve as reference) can be adjusted in two directions, more specifically in case of a drum plotter along the drum axis of the plotter and normal to the drum axis, i. e. along the circumference of the drum. In the following, these two directions will be referred to as the x and y directions, respectively. These directions are defined on the recording surface, e. g. the recording paper, in a similar manner. With these coordinates and an appropriately chosen origin, the position of each pixel of the image can be defined by its x and y coordinates.
It has been described in U.S. Pat. Nos. 3,596,275 and 3,916,421 that the drops, into which a continuously ejected ink jet disintegrates, can be electrically charged by applying a suitable voltage between the ink liquid in a conduit leading to the nozzle from which the jet issues, and a control electrode. If a DC voltage is used for charging, all drops will be equally charged. If the mass of the drops is kept constant by mechanical stimulation of the jet by an ultrasonic transducer as taught by U.S. Pat. No. 3,596,275, these equally charged drops of equal masses will be deflected by an equal amount by an electric deflection field established in a space between a pair of deflection electrodes through which the drops propagate toward the recording medium.
In the ink jet recorder described in U.S. Pat. No. 3,916,421 uncharged drops can proceed to the record surface in a "print" or "on" mode of operation, while sufficiently charged drops are deflected by the deflection field into a gutter and removed by suction (in other printers, the charged drops print and the uncharged drops are intercepted).